This invention relates to an ion implanter of the type rotating a rotary disc with a number of substrates fixed thereon to expose the substrates in succession to an ion beam.
Ion implanters have been used in producing semiconductor devices or liquid crystal display (LCD) devices. A conventional ion implanter includes a rotary disc having a number of substrates fixed thereon. During ion implantation, the rotary disc is rotated and reciprocated to expose the substrates in succession to an ion beam. Positive charges are induced in the substrate surface exposed to the ion beam. If the positive charges are accumulated to an excessive extent, a crack or a dielectric breakdown will occur in the substrate surface. In order to avoid such crack or dielectric breakdown, it is required to monitor the charge-up phenomenon in terms of the charges accumulated in the substrate surface by measuring the surface potential during the ion implantation. However, the conventional ion implanter employs a single surface potential sensor and cannot measure the surface potential with high accuracy.